The present invention relates generally to Code Division Multiple Access (CDMA) communication systems and more particularly to radio communication systems such as cellular, satellite or Personal Communications Networks which use both CDMA and Time Division Multiple Access (TDMA) for transmission. The invention may also be applied to other transmission media such as wireline Local Area Networks where it is desired to support many simultaneous communications links between subscribers on the network.
It is well known in the art that CDMA techniques may be used to transmit many independent signals that overlap in the same frequency spectrum. CDMA comprises coding information bits with a high degree of redundancy such that a much greater number of bits, known as "chips", are obtained for transmission.
The simplest form of redundancy comprises repeating a data bit many times, but CDMA comprises further the pseudo-random alternation of the sign or polarity of each of the repetitions using a code known to both transmitter and receiver. Reception of such a signal comprises undoing the sign alternations using a local replica of the code, and then combining the repeated bits using, for example, majority vote. Since an unwanted, overlapping and potentially interfering signal having a different sign alternation pattern will not be restored to repeated bits of like sign when the signs are undone with an incorrect code, such interfering signals will give, in principle, a net contribution of zero to the majority voting process and will therefore not cause errors.
When other signals having incorrect codes give exactly zero contribution to the majority vote process, that is, after undoing the sign alternation of a wanted signal the unwanted signals have exactly half their repeated bits of opposite sign to the other half, then such signals are called "orthogonal".
Orthogonal codes may alternatively be employed for coding a block of N data bits together to produce a representative block codeword having 2 to the power N-1 or N bits. Such codes are called "bi-orthogonal" and "orthogonal" block codes, respectively. When orthogonal or bi-orthogonal codes are used to discriminate between different transmitted data bit blocks from the same transmitter, they cannot also be used to discriminate between different transmitters. All or part of the power of the code may be used to code data bits from the same transmitter and then the remaining power may be used to discriminate between different transmitters. Telecommunications Industry Association (TIA) standard IS95 is an example of using orthogonal codes to discriminate between different data bit blocks (the IS95 uplink) and also using orthogonal codes to discriminate between different transmissions (the IS95 downlink).
Unfortunately, the number of available orthogonal codes for constructing a set of orthogonal signals is limited to at most the number of chips employed in the codeword. If a larger number of overlapping signals than that is desired, their codes can not all be mutually orthogonal. Moreover, orthogonality is destroyed by the propagation phenomenon, common in mobile radio propagation, known as multipath propagation or time-dispersion. Multipath propagation results when the path between a transmitter and a receiver comprises reflections from large objects, giving rise to echos with different delays. Codes that remain orthogonal when delayed or time-shifted with respect to each other cannot easily be constructed according to the prior art. Multipath echos that are delayed by one chip period or multiple chip periods are normally referred to as "independent rays".
Multipath echos of delay shorter than one chip period are not received with one or more whole-chip time shifts relative to the unshifted code, but give rise to another phenomenon known as Rayleigh fading. While such echos may only be a fraction of a chip period, they can be delayed by several whole and fractional cycles of the radio carrier frequency, which is generally of much higher frequency than the chip rate and therefore of much shorter wavelength.
These echos may therefore combine constructively or destructively depending on their phase, which can change rapidly due to receiver or transmitter motion. Thus the amplitude of a ray bearing the code shifted by one or more whole chip periods appears to vary randomly in amplitude and phase due to being composed of many smaller rays of delays shorter or longer than the whole number of chip periods.
A signal comprised of echos of various delays that are not necessarily multiples of a chip period can be exactly represented mathematically by a number of rays that are relatively delayed by exact multiples of the chip period, but which are Rayleigh fading in a more or less uncorrelated manner. The mathematical representation in this way can be regarded as collecting all the echos that lie within .+-.1/2 a chip period of an exact chip period delay multiple together to determine the amplitude and phase variation of a representative ray with that exact multiple chip delay.
Rayleigh fading, for slow speeds, can cause a ray to fade out for periods too long to be bridged by time-interleaved coding or other countermeasures, causing temporary loss of transmission for short periods and therefore errors in the transmission of information. If the signal can be represented by several rays of different whole-chip delay multiples and fading in an uncorrelated fashion, then the chance of all rays fading out completely is reduced, and fewer errors result. Thus multipath echos of multiple-chip delays can be beneficial in bringing about this so-called "path diversity gain". Unfortunately, as has already been stated, such echos have in the prior art had the disadvantage of denying the benefit of orthogonal codes.
If the chip period is reduced, there is a greater probability that echos will be delayed by one or more chip periods and each chip period will encompass a smaller number of echos in general. Ultimately, each individual echo or delayed path is resolved when the chip period becomes sufficiently short, and since each ray then consists of a single path, it does not exhibit the Rayleigh fading phenomenon. Unfortunately, if the environment encompasses a large number of such rays, receiver complexity to process the signal becomes excessive.
The U.S. military communications system known as JTIDS (Joint Tactical Information Distribution System) is another example of a system employing orthogonal codes to discriminate between different transmitter data blocks, as does the cited TIA standard IS95 in its uplink direction. IS95 transmits 64-bit scrambled codewords each carrying 6 bits of information whereas JTIDS transmits 32-bit scrambled codewords, each carrying 5 bits of information. IS95 transmits codewords in a continuous stream and employs means to counter multipath propagation known as a RAKE receiver, which will be described further below. JTIDS, on the other hand, time-compresses each single codeword for transmission in a single burst, and does not employ a RAKE receiver to combine multipath rays.
JTIDS is not configured as a network of base station each communicating with a plurality of mobile stations, but envisages a plurality of autonomous mobile or fixed stations that communicate directly with each other in pairs.
JTIDS is also not considered to be a direct sequence CDMA system that allows many users to overlap at the same time in the same frequency channel, as the 32,5 orthogonal outer code does not have the power to tolerate significant permanently overlapping interference. Instead, it uses frequency hopping to minimize the probability of clashes with other users. It therefore belongs in the class of frequency hopping spread spectrum systems and not in the class of direct sequence CDMA systems.
Furthermore, JTIDS receivers do not envisage time-expanding received bursts for processing as narrowband CDMA signals, using for example multi-user demodulators such as interference subtraction or joint demodulation, but rather directly process the wideband signal to decode a 32,5 orthogonal codeword to obtain a 5-bit Reed-Solomon symbol. Indeed, as a military system, JTIDS maintains security by keeping the codes of some user groups or pairs secret from other stations, so that compromise of a code would not compromise the security of all communications.
The security doctrine practiced by such military systems therefore prevents or teaches away from the techniques of joint demodulation which can benefit civil communications systems through making all CDMA access codes public.
The RAKE receiver is the name given to a prior art receiver adapted to process signals received via several relatively delayed paths. Such a reception channel is known as a multipath channel, and the different paths may be referred to as rays or echos. The RAKE receiver, together with innovative variations adapted more specifically to the cellular CDMA channel from base station to mobile station, are described in commonly owned U.S. patent application Ser. No. 08/187,062, entitled "A Method and System for Demodulation of CDMA Downlink Signals", filed Jan. 27, 1994, which is hereby incorporated by reference. It is explained therein how a receiver can isolate and then combine individual rays using correlation. If the receiver cannot isolate and combine all rays due to complexity limitations, then those that are not isolated and combined each represent a complete copy of the interfering signal environment, effectively multiplying the number of apparently overlapping interfering signals. Since any CDMA system places limits on the number of independent overlapping and interfering signals that can be tolerated without excessive transmission errors, unutilized echos cause a reduction in the number of signals that can be transmitted, i.e. in the capacity of the system measured in Erlangs per Megahertz per unit area.
U.S. Pat. Nos. 5,151,919 and 5,218,619, respectively entitled "Subtractive demodulation of CDMA signals" and "CDMA Subtractive Demodulation" describe novel means to increase the number of non-orthogonal CDMA signals that can be permitted to overlap, by decoding the strongest of the overlapping signals first and then subtracting it and its echos out before continuing to demodulate the next strongest signal, and so on until a wanted signal is decoded. The two above-cited patents by the same inventor as the present application and assigned to the same assignee are specifically incorporated herein in their entirety by reference.
Using subtractive demodulation according to the three above incorporated patents it can be shown that the amount of computation effort needed in a receiver increases with at least the cube of the chiprate, if the CDMA system is exploited to the full capacity of which it is capable. This means that the benefits of subtractive demodulation are most easily obtained for narrowband, low chiprate CDMA systems, causing low chiprate CDMA systems to exhibit better performance than high chiprate systems that cannot use the subtractive technique due to complexity limitations.
Thus, using the above techniques, it is difficult to simultaneously achieve the advantages of: 1) orthogonality, which is only available in the absence of time dispersion or echos delayed by one or more chip periods; 2) path diversity, which is only obtained when echos delayed by one or more chip periods are present; 3) resolution of individual rays to eliminate Rayleigh fading, only obtained with very high chiprates, on the order of 10 MB/s; and 4) interference subtraction, complexity limited to low chiprates, for example under 300 KB/s.
TIA standard IS95 specifies continuous CDMA transmission using a chiprate of approximately 1 MB/s, and this falls between two methods in being too narrowband to achieve the benefits of eliminating Rayleigh fading on the individual rays while being too high a chiprate and therefore too onerous for a low-cost, low-power mobile station to achieve the benefits of interference subtraction.
One method of extending the benefits of subtractive demodulation to higher chiprates is described in commonly owned co-pending U.S. patent application Ser. No. 08/570,431 entitled "Reorthogonalization of Wideband CDMA Signals", filed Dec. 11, 1995, which is assigned to the same assignee and is hereby incorporated by reference. This application discloses despreading signals in signal strength order to obtain narrowband signals, which are then notched out by zeroizing a frequency domain component using a narrowband notch filter. This technique is also used to null out delayed echos of a signal and subtraction errors by repeating the zeroizing process after first zeroizing other unwanted signals.
The aforedescribed re-orthogonalization principle applied by way of spectral nulling is illustrated in FIGS. 1 and 2. In FIG. 1, a receiver 100 downconverts the received signal, if necessary to a suitable intermediate frequency. The intermediate frequency is then despread using the code C1 of the strongest signal in despreader 101. The narrowband, despread signal is then nulled out in the spectral domain by nulling filter 102. The residual signal is then respread with code C1 in respreader 103 prior to being despread in 104 with code C2, nulling out signal 2 in filter 105 and respreading with C2 in block 106. According to one embodiment, re-orthogonalization of the signal with respect to C1, i.e. by subtracting out again a component that correlates with C1 after having subtracted or nulled out other signals, is shown as a second C1 despreader 107, second nulling filter for C1-correlated components 108 and second C1 respreader 109. After the resubtraction stage represented by blocks 107, 108 and 109, the residual signal can be further processed to extract other signals, and later resubtraction of C2 and C1 for a third time. Indeed resubtraction of any or all of previously subtracted signals may be performed to prevent accumulation of subtraction imperfections that hinder the decoding of weak signals.
FIG. 2 illustrates that some of the signal removal stages can be used for removal of differently-delayed rays of the same signal by using a delayed version C1-.sub.t-T of the code sequence C1.sub.t. Rays are preferably removed in descending signal strength order. For example, assuming ray 1 of signal 1 is the strongest received ray of all; then it is despread in a first stage 91 using Code C1.sub.t. The despread components of the rays of the same signal (e.g., signal 1 ray 1, signal 1 ray 2, etc.) may be fed to combiner 95 which may be, for example, a RAKE combiner, that tracks the phase and amplitude of every ray and performs coherent combination with the aid of complex weights to enhance the signal for decoding in decoder 96. Block 95 can alternatively be a selection combiner for selecting for decoding always the strongest ray of signal 1, which, however, should always be arranged to be that removed in stage 91 by using the appropriate code delay C1.sub.t, C1.sub.t-T, etc. in stage 1. Block 92 illustrates that rays of other signals may be despread and removed before a second ray of signal 1 is despread, which is desirable if the other signal rays are stronger than signal 1 ray 2.
Signal ray 2 is despread in stage 93 by using code C1 delayed by T, i.e., the code sequence C1.sub.t-T where T is chosen to correspond as closely as possible to the delay of the second strongest ray of signal 1 relative to the strongest ray of signal 1. The despread ray 2 component is fed to combiner 95 before being filtered out from the signal passed to subsequent stages represented by block 94. Block 94 can proceed to despread and remove other rays of signal 1, rays of other signals, or to re-subtract components correlated with any of code C1.sub..sub.t, code C1.sub.t-T or any other code or delayed code used previously in an earlier signal removal stage.
Wideband re-orthogonalization according to the above disclosure can be carried out by analog filters which are less power-consuming than digital signal processing; however, the number of analog filters that can be practically included in a receiver such as a mobile phone is limited to a much smaller number than could be afforded in a cellular base station for example, and so the technique is more practicable to the CDMA uplink than to the CDMA downlink.
Another practical limitation of wideband CDMA for duplex communications systems is interference between own transmitter and own receiver. Such interference may be prevented in narrowband FDMA, TDMA or CDMA systems by allocating a separate frequency or frequency band for transmission and reception respectively by a portable phone, the transmit/receive frequency allocations being reversed at the base station. The frequency spacing between transmit and receive frequencies is known as the duplex spacing. A typical duplex spacing used is 45 MHz. Unfortunately when wideband CDMA is employed, the duplex spacing may be insufficient in relation to the signal spread bandwidth to prevent the transmitter's spectral tails from extending into the receiver band and thereby causing interference.
The above deficiencies of IS95 and other CDMA systems in hindering the respective benefits of wideband and narrowband CDMA systems from being achieved simultaneously are overcome when practicing exemplary embodiments of the invention that will now be described.